Brushless DC motors have been well known in the art for approximately two decades. Generally, brushless DC motors are smaller but are more costly than both AC motors and conventional DC motors of equal horsepower (HP) ratings. However, the initial increase in the cost of these motors is offset by reduced maintenance costs over the life of the motor. Additionally, due to the fact that control circuitry can be an integral part of the motor, brushless DC motors offer both little or no maintenance, and would also allow small signal control of the rotating speed of the rotor. Furthermore, since brushless DC motors do not, of course, utilize brushes, they do not create brush arcing and thereby eliminate electromagnetic interference (EMI) and explosion hazards. Additionally, brushless DC motors offer a reduction in acoustic noise, often due solely to brush friction, and do not produce carbon residue or gaseous by-product associated with conventional motor operation. Brushless DC motors are generally more efficient than conventional DC motors and have a fairly constant torque versus input current characteristic. Finally, brushless DC motors have a lower servo-time constant, and thus provide a more rapid output response. The combination of the foregoing list of advantages makes brushless DC motors suitable for submerged operation, operation in a combustible atmosphere, or operation in a high vacuum.
There are three basic types of brushless DC motors. A first type is known as the DC/AC Inverter Brushless DC motor. This type is characterized by the fact that it can be operated only from a DC power supply. Furthermore, brushless DC motors of this type employ an AC servomotor incorporating an electronic inverter energized by the same DC power supply.
A second type of brushless DC motor is known as the Limited Rotation Brushless DC motor. This motor is unique in that it is not intended for continuous rotation in ordinary use, and it can only provide an output torque over a partial radius of operation, typically .+-.90.degree. maximum. Output rotation may be counterclockwise (CCW) or clockwise (CW) dependent upon the polarity of the direct current energizing its stator coil.
The third type of brushless DC motor is the Electronic Commutation Brushless DC motor. This type of brushless DC motor is distinguished from the other two types of brushless motors by the utilization of a wound torque inducing coil, a permanent magnet (PM) rotor and a rotor position sensor which serves to energize the torque inducing coil in synchronization with the rotor movement. Proper synchronization with rotor movement eliminates the need for commutator and brush assemblies found on conventional motors. The rotor position sensor is usually electronic in nature and incorporates high speed switching transistors. The high speed switching transistors can be activated by any number of devices that are capable of "sensing" the rotor's position. Devices contemplated for this purpose can be cam shaped light shields or other photoelectric sensor arrangements secured to the rotor shaft, magnetic transducers, Hall Effect devices, piezoelectric crystal transducers, electrostatic sensors or electromagnetic induction coils. The sole purpose of the rotor position sensor is to provide a signal upon which the transistor switch can become activated.
U.S. Pat. No. 3,662,196 to Ruschmann discloses a brushless DC motor incorporating a fixed permanent magnetic field interacting with a second movable magnetic field that changes polarity, such as an armature winding. Current is carried from an external circuit to the armature winding via switches arranged in two circular arrays and equiangularly spaced within each array. This patent involves the use of many parts that are expensive to manufacture and require labor intensive methods of assembly.
U.S. Pat. No. 4,517,477 to Pankratz describes the use of one or more permanent magnets to form a rotor, wherein each magnet has first and second poles of opposite polarity. The magnets are arranged such that alternating first and second poles are spaced angularly, relative to the rotor. A magnetic device defining a stator alternates between a first phase attracting the first pole of a given magnet, and a second phase repelling the second pole of the given magnet. Timing devices detect the position of the magnet and signal the magnetic device for alternating between the first and second phases. A solenoid, responsive to the timing means, selectively and alternately positions the stator magnets in close proximity to the rotor for alternately attracting or repelling the rotor magnets. The Pankratz patent employs an elaborate and precisely formed rotor, requiring that the rotor have a groove or track through which the stator magnet is seated. Furthermore, the rotor magnets, which are commercially available bar-type magnets, must be reformed into an arcuate pattern in order to function properly on the rotor periphery.
U.S. Pat. No. 3,688,136 to Salverda discusses a brushless DC motor having a rotor carrying permanent magnets at its outer ends. These magnets are repulsed by pusher magnets that are oriented in the same direction as the rotor magnets. Cam operated solenoids interject the pusher magnets into close proximity behind the rotor magnets. The Salverda patent requires many moving parts and elaborate circuitry for operating the cam timing means and solenoid linkage. These shortcomings represent an undesirable drain on the DC power supply of the motor, a costly sacrifice to the operational advantages of a brushless DC motor.